JAKARTA -- The discovery of Avian influenza in cats and dogs has heightened concerns about a virus that experts had thought was basically infecting chickens, ducks and other fowl.

Health experts have called for closer monitoring of the H5N1 virus in domestic animals after Indonesian scientists detected it in stray cats near poultry markets in some parts of the country.

They worry that if the virus adapts to mammals it could more easily spread among people.

A survey by Chairul Anwar Nidom, a scientist at Airlangga University in Surabaya, found H5N1 antibodies in 20 percent of 500 stray cats near poultry markets in four areas in Java, including Jakarta, and one area in Sumatra where there had been recent human H5N1 cases or outbreaks of the disease in poultry.

The survey said the findings suggested the cats had probably been infected because they ate infected poultry.

In another case, Gusti Ngurah Mahardika, a virologist at Udayana University, surveyed pigs and domestic animals in Bali between September and December last year and found the virus in two dogs and a cat.

Although the cases in cats and dogs are not widespread, scientists are concerned.

"With more species of mammals infected, that could be a sign that the virus is mutating to adapt to mammalian hosts. If they are adapting to mammals, they could be on the way to adapting to humans, to become a human virus," Lo warned.

PIGS, TOO

Musni Suatmodjo, Indonesia's animal health director, said there had been reports about the virus in cats and pigs in Indonesia, but had no details.

"Informally, there's information that bird flu infection in cats was found in Bandung and Bali. We also found another case in pigs in Yogyakarta," he told Reuters.

The Indonesian surveys added to concerns about the virus which has killed 164 people since late 2003 and has flared again with a string of human H5N1 cases in Indonesia and the virus spreading among poultry in Vietnam, Thailand and Japan.

While the discovery of the virus in felines is not new, it is not known how efficiently domestic animals can pass the virus on to other mammals, such as humans.

Experts fear the H5N1 virus, which still remains mainly a virus of birds, could change into a form easily transmitted from person to person and sweep the world, killing millions within weeks or months.

Yong Poovorawan, a virologist at Thailand's Chulalongkorn University, detected the virus in a dog in October 2004 in Suphanburi province, 100 km (60 miles) north of Bangkok.

He also detected the virus in more than 50 tigers at the Sri Racha zoo in the eastern city of Chonburi in the same month, but found no evidence of the virus in any of its zookeepers.

"We have no evidence that cats or dogs can pass the virus to humans but we have evidence of probable tiger-to-tiger transmission due to the close contact among tigers," he said.

"But we have to be concerned about the virus in avians, humans or any other species because it is a highly pathogenic virus. We have to be alert to it."

In Hong Kong, where the virus made its first known jump to humans in 1997, the government said it detected the virus in a peregrine falcon and a house crow, bringing to seven the number of wild birds found dead with the virus since December 2006.

Lo called for a stop to the import of wild birds from China for release in Hong Kong, a common ritual among Buddhists.

"We should be very careful about capturing wild birds and releasing them into a foreign environment. That could bring the virus from one area to another," Lo said.

In Taiwan, government researchers announced on Monday that an experimental H5N1 vaccine for humans had produced antibodies in small animals. They hope to complete two clinical trials before the end of 2008. They gave no details about the animal test.